1. Field of the Invention
Exemplary embodiments of the present invention relate to a turbine, and more particularly, to a turbine in which rotor blades (buckets) are detachably fixed to a rotor wheel.
2. Description of the Related Art
A steam turbine is an apparatus which converts kinetic energy into rotational force by rotating blades using high-temperature and high-pressure steam generated by a large boiler for a power plant. The steam turbine is classified into a high-pressure turbine, a medium-pressure turbine, and a low-pressure turbine and maximizes efficiency.
FIG. 1 is a perspective view illustrating an example of buckets according to the related art. Each bucket 10 includes a vane 11, a shroud 12 formed at a radial outer end portion of the vane 11, and a dovetail 13 formed at a radial inner end portion of the vane 11.
The dovetail 13 is a component for fixing the bucket 10 to a rotor wheel 14. The dovetail 13 may be classified into (1) a tangential entry type, (2) an axial entry type, (3) a pinned finger type, and (4) a keyed axial entry type, according to a manner of coupling the dovetail 13 to the rotor wheel 14. In the types of numbers (1) and (3), the dovetail is tangentially (or circumferentially) inserted and coupled to the rotor wheel. In the types of numbers (2) and (4), the dovetail 13 is axially inserted and coupled to the rotor wheel 14.
The dovetail 13 shown in FIG. 1 is an axial entry type dovetail. Referring to FIG. 1, dovetail grooves 15 are circumferentially formed at intervals on a circumferential portion of the rotor wheel 14. Each of the dovetail grooves 15 has a cross-sectional shape in the form of a corrugation at both axial sides thereof based on a radial cross-section thereof. In this case, the dovetail 13 of the bucket 10 also has a shape corresponding to the dovetail groove 15. That is, the dovetail 13 and the dovetail groove 15 have a male and female coupling relation.
In a method of assembling the bucket 10 in which the axial entry type dovetail 13 is applied, the bucket 10 integrally includes the shroud 12, the vane 11, and the dovetail 13, and the bucket 10 is axially inserted and assembled to the dovetail groove 15 using the dovetail 13 along the circumferential portion of the rotor wheel 14.
In the conventional method of assembling the bucket 10 of the steam turbine in which the axial entry type dovetail 13 is applied, there is however a problem in that it is impossible to assemble a second bucket 20 which is finally assembled since the second bucket 20 interferes with the adjacent bucket 10 (the shroud 12, a platform 11a, and the vane 11).
In this regard, U.S. Pat. No. 6,030,178 discloses a method of opening adjacent buckets 10 in opposite directions (a tangential direction; {circle around (1)}) and then inserting a second bucket 20 in a radial direction ({circle around (2)}) so that the second bucket 20 is seated and installed to a rotor wheel 14. Finally, a so-called Caruso key 16 is simultaneously inserted and coupled to a dovetail groove 15 of the rotor wheel 14 and a dovetail groove 21 of the second bucket 20 in an axial direction ({circle around (3)}).
However, the above related art has the following problems.
First, in order to insert the Caruso key 16, the existing dovetail (a protruding portion) should be cut and the dovetail groove 21 should be separately formed on a platform (a root portion) 11a of the second bucket 20, thereby increasing the sizes of the buckets 10 and 20. Thus, there are problems in that centrifugal stress of the buckets 10 and 20 is increased and a consumed bucket material is increased.
Secondly, since the Caruso key 16 is made of an inconel material so as to withstand high centrifugal stress, it has heat transfer properties different from the bucket made of a steel material. Therefore, due to excessive thermal stress caused by a difference in thermal expansion at hot parts of the key, there may be a limit in terms of a design. In addition, since the key itself has a complicated shape, the key may have poor machinability and material costs thereof may be increased.